1. Technical Field of the Invention
The present invention relates to a method of oxidation of cyclohexane to adipic acid, and more particularly to the processing of the reaction mixtures obtained from this oxidation reaction.
2. Description of the Prior Art
The direct oxidation of cyclohexane to adipic acid is a process which has been explored for a long time, in particular on account of the obvious advantages there would be in converting the cyclohexane into adipic acid in a single step and without using an oxidizing agent such as nitric acid, since this compound generates nitrogen oxides which need to be processed subsequently in order to avoid pollution.
Patent Application WO-A-94/07833 describes the oxidation of cyclic hydrocarbons into corresponding diacids, in a liquid phase containing a solvent, at a temperature of at least 60.degree. C., using a gas containing oxygen and in the presence of an oxidation catalyst such as a cobalt compound, and points out that the solvent represents less than 1.5 mol per mole of cyclic hydrocarbon, that the said solvent comprises an organic acid having only primary or secondary hydrogen atoms and that the reaction is carried out in the presence of at least 0.002 mol of cobalt-based catalyst per 1,000 g of reaction mixture. At the end of the reaction, the diacid formed is isolated.
Patent Application W0-A-94/07834, filed on the same day as the one above, also describes the same process, but develops the phases of processing the final reaction mixture. This processing consists in separating out the diacid formed, by cooling the reaction mixture in order to cause the said diacid to precipitate out, in separating by filtration the diacid from two liquid phases, a non-polar phase which is recycled, and a polar phase which is also recycled after an optional hydrolysis and separation of an additional amount of diacid.
These patents present solutions which allow the one-step oxidation of cyclohexane to adipic acid with industrially acceptable selectivity, but they provide no solution which is industrially applicable to the processing of the reaction mixture obtained from the oxidation, taking into account the separation of the many different reaction products and side products, unconverted products and the catalyst.